1. Field of the Disclosure
The disclosure relates to a pulsed, high power optical system. In particular, the disclosure relates to a pulsed multi-cascade fiber optical system configured with at least one passive modulator which is operative to isolate the consecutive gain blocks of the optical system from parasitic optical signals.
2. Prior Art
One of techniques, allowing increasing the energy level of the output optical pulse generated by a pulsed-operated laser system, includes cascading multiple gain blocks or simply amplifiers. The gain factor of the cascaded laser system corresponds to the multiplication of the gain coefficients of all blocks which may be, thus, high. However, the high gain of the output may be unacceptable for the following reasons.
One of the reasons includes the presence of amplified spontaneous emission (ASE) which is proportionally amplified by the gain factor of the system and, thus, may reach unacceptably high levels. The consequences of high levels of noise—the ASE is no more than a useless signal or noise—include, for example, the pronounced inefficiency of a laser system. Still a further consequence is associated with the very presence of the high energy levels of the ASE at the output of the system which may be unacceptable in a variety of laser system's applications. For example, the presence of the high-energy ASE cannot be tolerated in marking sensitive materials, such as plastic.
A further reason explaining detrimental effects of high gain factor to the operation of the pulsed laser system includes a so-called self pulsing phenomenon causing the degradation and possible destruction of amplifiers or gain blocks, particularly those configured from fiber. Still another reason for avoiding high gain relates to a buildup of backreflection unavoidable in practical applications of the system. Concomitantly, backward Rayleigth scattering, Brillouin scattering as well as other scattering noise developed in the cascades and propagating through the laser system also contribute to the degradation of the system.
As a result, as readily realized by one of ordinary skills in the pulsed-operated laser art, the gain blocks should and are typically optically isolated. The means used for isolating cascades are briefly discussed immediately below.
In accordance with one approach, adjacent gain blocks may be mutually isolated by a bulk isolator. The bulk isolator is configured to transmit radiation in one direction and block it in the opposite direction. As a consequence, while the problem with backreflected noise may be adequately addressed, the forward propagation of noise still poses a problem.
The other approach includes the use of active modulators configured to manipulate properties of light beams, such as the optical power or phase. The active modulators include, among others, acousto- and electro-optic modulators. The former, as well known to the artisan, is based on the acousto-optic effect, while the latter exploits the electro-optic effect. The active modulators thus operate as a synchronously timed gate employed between adjacent gain blocks. The active modulators have certain advantageous over bulk isolators. For instance, the forward absorption of the ASE to a subsequent, downstream gain block is prevented, except for a temporary open window of modulator. As a consequence, the resulting noise at the output of the laser system is substantially suppressed. However, the use of active modulators adds significant cost and complexity to the laser system and leads to a less robust and bulkier structure.
One of the solutions of the above-discussed problems includes the use of saturable absorber which is briefly mentioned by W. V Sorin and et al. in a paper titled “Single-mode-fiber saturable absorber” incorporated herein by reference in its entirety. However, to the best knowledge of Applicants, neither the incorporated paper, nor any other known to Applicants source suggests the optimization of the saturable absorber in a multi-cascaded high power laser system.
A need, therefore, exists for providing a cost-effective multi-cascaded pulsed-operated fiber laser system which is provided with an optimally configured saturable absorber capable to minimize the above-discussed problems.